The present invention relates to color image processing methods and apparatus that extract colors from color images. More specifically, the present invention relates to color image processing methods and apparatus that can extract achromatic colors in addition to two kinds of colors.
Various prior art techniques are well known to read a color document by means of a scanner and to extract colors from the color image which has been read.
(1) A first method
As illustrated in FIG. 1, this method employs a red light source 2 and a blue light source 3 to illuminate a color document 1. A photoelectrical conversion means 4, like a CCD, receives respective optical information and converts them to electrical signals. Output signals from the photoelectrical conversion means 4 are normalized based on the output value of white color paper, and designated as VR and VB, respectively. These two signals are operated to make a color extraction map. The 1107 report in the preprints for 1982 conference of Japan Electric & Communication Institute describes a color extraction map shown in FIG. 2, and suggests that a plurality of colors may be extracted based on this color extraction map. The abscissa axis of FIG. 2 is normalized output (%) from the photoelectrical conversion means 4 when the red light source is on. The ordinate axis is normalized output (%) from the photoelectrical conversion means 4 when the blue light source is on.
(2) A second method
This method employs two types of light detection means whose spectral sensitivities are different for one picture element. The outputs VA and VB from the light detection means are operated to extract colors. This method is disclosed in Kakai No. 57-44825. For example, the light detection means detects for the vertical luminance signal axis (VA+VB): ##EQU1## and for the transverse hue signal axis (log VA-log VB): ##EQU2## Where a1, a2, b1, and b2 are constants, respectively. FIG. 3 is a color extraction map obtained by this method.
(3) A third method
This method employs a plurality of dichroic mirrors, prisms, or R-, G-, B-filters to separate optical information into three colors of red, green, and blue. This method is disclosed in Japanese Patent Laying-Open Gazette (Kokai) No. 62320/1975.
FIGS. 4a-4h illustrates various methods to separate colors. The apparatus shown in FIG. 4-a is arranged so that colors on an aerial image 12 formed by a photographic lens 11 are separated into three colors by means of a plurality of relay lenses 13 through 16 and dichroic mirrors 17 and 17', and each color image is formed on a plurality of camera tubes 18 through 20, respectively. In the apparatus shown in FIG. 4-b, a plurality of specific shaped prisms 21 through 24 are arranged between the photographic lens 11 and the camera tubes 18 through 20, and the dichroic mirrors 17 and 17' and inserted, respectively, between the prisms 21 and 22, and between prisms 23 and 24, so that a color image through the photographic lens 11 is separated into three colors.
In the apparatus shown in FIG. 4-c, three prisms 25, 26, and 26' whose apex angles are an acute angle are brought together and the dichroic mirrors 27 and 28 are inserted between each prism to separate a color image into three colors. In the apparatus shown in FIG. 4-d, the prisms shown in FIG. 4-c are inverted. Dichroic mirrors 29 and 30 are formed at boundaries between each prism.
The above described conventional image processing methods and apparatus can provide means to separate colors. However, techniques for how to utilize the separated colors are still under development.
In view of the above described status and disadvantages of conventional image processing methods and apparatus, it is an object of the present invention to provide an image processing method and apparatus that can output well-balanced images.
The following are well-known as image processing methods which detect and output images having more than three colors.
(a) This method separates colored light from the input image into three elementary colors, R (red), G (green), and B (blue), according to the three-elementary color principle, and detects each light intensity by means of CCD image sensors or the like to obtain a color image. This method corresponds to the third method described above, and shown in FIG. 4-a to 4-d.
(b) This method employs a plurality of light detection devices, each of which is equipped with color filters of R, G, and B, or Y (yellow), M (Magenta), and C (cyan), or W (white), Y (yellow), and C (cyan), or G (green), Y (yellow), and C (cyan), and provides the thus obtained outputs to detect color images.
FIG. 4-e illustrates an example of arranging color filters employed in method (b). As illustrated in this figure, a plurality of color filters are arranged in this order of W (white), Y (yellow), and C (cyan), repeatedly. A photoelectric conversion element is provided on the central portion of each filter as illustrated by diagonal lines in the figure. All light passing through each filter is converted into electrical signals. These conventional methods (a) and (b) require light detection devices that have different spectral sensitivities for each picture element on the color document. Therefore, method (b) has the disadvantages discussed below.
(1) The thickness dispersions or uneven thickness of each color filter cause the dispersions of output sensitivities from the photoelectric conversion elements to increase. Therefore, the yield of the photoelectric conversion elements during manufacturing decreases.
(2) A lot of light receiving portions (number of picture elements) are needed to make images with high resolution of every color. The number of picture elements in the ready made, reduction optical system that employs a photoelectric conversion element covered with a filter may be substantially reduced and cannot satisfy the requirements described above. Therefore, a plurality of photoelectric conversion elements are required, thereby increasing complexity of the control operation. On the other hand, two methods are possible to satisfy the requirements described above by an equi-magnification optical system. One is to employ a configuration comprising a plurality of CCD chips. But joining each chip in this configuration is difficult and another process is required to obtain serial image.
The other is to use materials of a-Si or CdS-CdSe family. This method has the disadvantage of a low image formation speed.
Method (a) requires a plurality of image formation elements to provide high speed image formation. The image formation system, however, is very expensive and requires a lot of man-hours to be aligned. An image formation system that employs a single light detection device and a plurality of color light emitting sources is developed as one attempt to resolve these disadvantages. FIG. 4-f illustrates one example for configuring the image formation system. In this figure,
721 is a red/green light source LED; PA0 722, a blue fluorescent panel; PA0 723, an orthochromatic filter; and PA0 724, a cylindrical lens.
The reason why a blue fluorescent panel is used as a blue light source in place of an LED is that a blue LED with high efficiency is not available at present. FIG. 4-g shows a spectra example of the light emitting source employed in FIG. 4-f. The abscissa axis represents the wave length (nm), and the ordinate axis represents the relative intensity (%). R represents the spectrum of the red LED 721; G, the spectrum of the green LED 721; and B, the spectrum of the blue fluorescence panel.
These light sources sequentially emit light in accordance with the timing diagram shown in FIG. 4-h (a), and illuminates a document 726 placed on a glass plate 725. The reflected light from the document 726 enters into a SELFOC lens 727, passes through the lens, and is converted to electrical signals by a CCD sensor 728. When the optical information is converted to an electrical signal by the CCD sensor 728, a transmission pulse is outputted as shown in FIG. 4-h (b). As illustrated in FIG. 4-h (c), electrical charges in the CCD are taken out as a scanning output. As described above, color signals are obtained by using an apparatus shown in FIG. 4-f, lighting three color light sources of R, G, and B during the scanning of a line, followed by arithmetic processing. This image forming system provides a small-sized color image processing apparatus with low manufacturing costs. The apparatus, however, has disadvantages of a complicated image forming system, and speeding up the image formation is difficult.
In view of the foregoing, it is an object of the present invention to provide an image processing apparatus which has a relatively simple configuration and ensures a high speed image formation. More specifically, it is an object to provide an image processing apparatus which can separate colors easily. In order to copy, with sufficient density, the colored portion of red, blue, and the like on the color document, the developed portion is constituted so that the bias voltage can be controlled in a conventional monochrome, analog copying machine. For example, during normal development, the developed bias voltage is lowered to ground potential for a document of orange or light blue, which has a low reflection density, in order to improve the development ability of such color and to effect sufficient copy density. On the other hand, to obtain sufficient copy density of a document written with blue writing utensils (for example, a ball point pen), writing utensil manufacturers mix a small quantity of carbon black into the ink. When the document is written with black-, blue-, and red-series writing utensils at the same time, some red characters cannot be copied with sufficient density even though black and blue characters are copied with sufficient density. This problem is caused by the different reflection density due to the differences of colors. Therefore, in a digital copying machine which processes image information by photoelectric conversion and digital conversion, it may be preferred to apply a threshold value having different values corresponding to respective color ranges of colors to be reproduced so that every color image can be copied with sufficient density.